Radiation transducers, such as radiation detectors, convert radiant power to an electrical signal or other physical property that is then converted to an electrical signal. The other physical properties may include resistance, heat, or other measureable property. Radiation transducers generally are chosen for particular applications by ascertaining their properties, which can include sensitivity, dark current, impedance, noise, and frequency response. A detector's frequency response can be correlated to its ability to detect rapid changes in the radiation incident on the detector.
In many cases, it is desirable to operate radiation transducers in an alternating current (AC) mode, where the radiation being measured is modulated in time, usually with a chopper wheel that alternates the radiation by allowing or not allowing radiation to pass between the radiation source and a detector. Another approach to modulating the signal includes the use of electronic gating circuits on the output side of the transducer. Operating a radiation detector in AC mode allows for improved measurements by removing slow signal drifts. Typically, the modulation of the chopper wheel is consistent in its frequency, i.e., the chopping rate is constant. An optical system consisting of a light source, a chopper wheel, a detector, and optics to convey light from the source to the detector can be used with a transmission cell containing a chemical mixture to study the chemical mixture. With light input with constant amplitude, with a uniform chopper wheel, and with a detector having a very fast frequency response, a derived signal from the detector would ideally approach a rectangular wave signal, which quickly rises to a maximum value and falls to zero when the radiation is alternately allowed to pass the chopping mechanism.
In practice, most systems do not behave ideally to produce a perfect rectangular wave signal. Oftentimes, the optical system is chopped at a relatively high rate to move the systems frequency away from other noise sources, such as 60 Hz electrical noise, to allow for improved signal-to-noise (S/N) ratios. As a result, real detector signals resemble sinusoidal waveforms as the chopping speed approaches the detectors response frequency.